A method capable of separating components bonded together has been required since shortly after the first method for bonding these components together have been developed, and with each new component or bonding material, the requirements for a process capable of debonding that component/bonding material combination typically change. One relatively new type of component to be debonded is industrial ceramics which are part of a rapidly growing segment of the industrial market.
Ceramics have characteristics with regard to strength, density, and thermal properties which make them a very useful material. However, ceramics can also be very expensive because high purity material is often used in the processing of the ceramics and these materials occasionally require high-precision milling. Thus, the cost of fabricating a single ceramic component can exceed thousands of dollars. Because of the potentially high cost to manufacture, manufacturers and resellers desire to reduce manufacturing costs by recycling these components.
One of the first steps in recovering a ceramic component is to separate the ceramic component from other components attached to it. The ease in separation of these components from each other typically depends upon the type of bonding used to combine them. For example, a mechanical connection is typically easier to disassemble than an adhesive bond. However, an adhesive bond is the type of bond typically used with ceramics.
Traditionally, three general methods have been used to separate adhesively bonded components, and include applying mechanical force, chemical dissolution, and conventional heating. However, these methods each have several disadvantages associated with them. Use of mechanical force is the oldest method to separate components that are adhesively bonded. However, because the adhesive is typically designed to prevent the components from being separated by a mechanical force, the mechanical force required to separate the components can be very destructive to the components themselves. Ceramic components, in particular, are very susceptible to damage from mechanical force because ceramics in general tend to be brittle. Also, even if the components are separated intact, the force of the separation may introduce microstructural surface defects into the ceramic components, and these defects have the potential to cause the component to fail at a later time. Additionally, even if the components are separated successfully, separation by mechanical force still leaves an adhesive residue on the components. This adhesive residue often must be removed before the component can be reused. This additional step adds to the cost of recycling the components and presents another opportunity for the components to be damaged.
A second process used to debond components is to chemically dissolve the adhesive. This process involves applying a solvent so as to dissolve the adhesive. One difficulty with this process is that some portion of the adhesive may not be readily accessible to the solvent. For example, with two large flat pieces bonded together on their flat sides, the adhesive in the very middle of the bond will not be dissolved until the time-consuming process of dissolving and removing all the adhesive surrounding it is completed. Another problem associated with chemical debonding is the waste stream generated from the adhesive being chemically dissolved. This waste stream is typically considered a hazardous material, and the proper disposal of this waste steam increases the costs of the recycling process. Even costs associated with disposal of a non-hazardous waste stream negates some of the benefits associated with recycling components. Still another problem with the use of a solvent is that the solvent may attack the components as well as dissolve the adhesive. This attack on the components may degrade the usefulness of the components, and thus negate the benefit of the recycling process.
A third method of component separation is to use conventional heating. With conventional heating, the entire bonded assembly is heated to at least the temperature at which the adhesive loses its bonding properties. Once the adhesive has reached a debonding temperature, the components can be separated. One problem with this method is the length of time required to complete the process which is a result of heat transfer characteristics inherent with conventional heating.
With conventional heating, the components must first be heated, and then the components conduct that heat to the adhesive. However, ceramics in general have characteristics that make this process very inefficient. First, ceramics are typically poor conductors of heat. Thus, the heat applied to the ceramic takes a long time to reach the adhesive. Second, ceramics are typically excellent absorbers of heat. Thus, a large amount of heat is needed to raise the ceramic to the debonding temperature of the adhesive. Also, once the ceramic is heated to the debonding temperature, the ceramic requires a long time to cool when the heat is removed, which makes immediate handling of the ceramics difficult. Thus, this particular process has the disadvantage of being time consuming and energy intensive. Also, the large amount of heat applied to the components may damage the components because a long period of time at high temperature can cause detrimental microstructural changes such as grain growth.
Although microwave energy has not been used to cause an adhesive to reach a state in which the adhesive loses its bonding properties, microwave energy has been used with adhesives and to separate components. For example, U.S. Pat. No. 5,644,837 to Fathi et al. discloses applying microwave energy to cure a thermoplastic or thermosetting resin. Another example of microwave use is disclosed in U.S. Pat. No. 5,675,909 to Pare. Pare discloses a process for accelerating the separation of volatiles from liquids or solids using microwave energy. However, neither of these references address the problem of separating two components that have been combined with an adhesive.